Multi-axis magnetic lens for focusing a plurality of charged particle beams

ABSTRACT

The present invention provides two ways to form a special permeability discontinuity unit inside every sub-lens of a multi-axis magnetic lens, which either has a simpler configuration or has more flexibility in manufacturing such as material selection and mechanical structure. Accordingly several types of multi-axis magnetic lens are proposed for various applications. One type is for general application such as a multi-axis magnetic condenser lens or a multi-axis magnetic transfer lens, another type is a multi-axis magnetic non-immersion objective which can require a lower magnetomotive force, and one more type is a multi-axis magnetic immersion objective lens which can generate smaller aberrations. Due to using permeability-discontinuity units, every multi-axis magnetic lens in this invention can also be electrically excited to function as a multi-axis electromagnetic compound lens so as to further reduce aberrations thereof and/or realize electron beam retarding for low-voltage irradiation on specimen.

CLAIM OF PRIORITY

This application claims the benefit of priority of U.S. provisionalapplication No. 61/577,888 entitled to Weiming et al. filed Dec. 20,2011 and entitled “Multi-axis Lens Magnetic for Focusing a Plurality ofCharged Particle Beams”, the entire disclosures of which areincorporated herein by reference.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S.application Ser. No. 13/464,261, filed May 4, 2012. This application isrelated to co-pending U.S. application Ser. No. 12/968,221 entitled toChen et al. filed Dec. 14, 2010 and entitled “APPARATUS OF PLURALCHARGED PARTICLE BEAMS WITH MULTI-AXIS MAGNETIC LENS”, the entiredisclosures of which are incorporated herein by reference.

This application is also related to co-pending U.S. application Ser. No.12/968,201 entitled to Chen et al. filed Dec. 14, 2010 and entitled“APPARATUS OF PLURAL CHARGED PARTICLE BEAMS WITH MULTI-AXIS MAGNETICLENS”, the entire disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-axis magnetic lens and variantsthereof used for focusing a plurality of charged particle beamsindividually and in parallel. More particularly, it relates to amulti-axis magnetic lens acting as an objective lens or a condenser lensor a transfer lens in a multi-beam apparatus which uses a plurality ofelectron beams to in parallel expose patterns onto or inspect defects ona wafer or mask in semiconductor manufacturing industry. Compared with aconventional single-beam counterpart of the multi-beam apparatus, it canobtain a much higher throughput without degrading spatial resolution.

2. Description of the Prior Art

In semiconductor manufacturing industry, an electron beam has been usedto expose patterns onto or inspect defects on a wafer or a mask whencritical feature dimensions of patterns or defects are beyond thecompetent ability of a photon beam. The reason is that an electron beamcan offer superior spatial resolution compared to a photon beam due toits short wavelength. However, such a superior spatial resolution willbe fundamentally deteriorated by electron interaction or called asCoulomb Effect as the electron beam current is increased to obtain athroughput competent for mass production.

To mitigate the limitation on throughput, instead of using one electronbeam with a large current, it is proposed many years ago to use aplurality of electron beams each with a small current to expose patternsonto a wafer in parallel, such as U.S. Pat. No. 3,715,580. Forstructuring a multi-beam apparatus using a plurality of electron beams,one critical problem is how to separately focus multiple electron beamsindividually and in parallel. Configuring multiple conventionalsingle-beam columns (MSCs) into one multi-beam apparatus was a firstsolution naturally thought out. Because the spatial interval betweenevery two adjacent beams must be large enough to physically accommodatetwo single-beam columns in parallel, the number of electron beamsavailable for a wafer or a mask is not sufficient for mass production.As an alternate to using the MSCs, configuring a multi-axis lens toindividually focus multiple electron beams in parallel is a promisingway to be able to use more electron beams. Compared with the structureof the MSCs, a multi-axis lens will reduce the beam interval by 50%,thereby almost doubling the apparatus throughput.

In U.S. Pat. No. 3,715,580, Maekawa et al. propose a multi-axis magneticlens for throughput improvement of an IC pattern exposure system. Themulti-axis magnetic lens is schematically shown in FIG. 1A, whichcomprises a common excitation coil 44, one yoke 43 and two magneticconductor plates 41 and 42 with a plurality of through round holes inpairs. When an electric current is exerted into the coil 44, a magneticround-lens field will be generated between each pair of coaxial holesrespectively in the plates 41 and 42. By this means, multiple magneticsub-lenses are therefore formed, such as 10, 20 and 30. Each magneticsub-lens has an optical axis coincident with the coincident central axesof two coaxial holes (such as 31 in FIG. 1B), and can focus an electronbeam (such as 1 in sub-lens 10, 2 in sub-lens 20 and 3 in sub-lens 30)entering the sub-lens along the optical axis thereof.

In the foregoing multi-axis magnetic lens, the magnetic flux leakagebetween each pair of coaxial holes depends on the positions thereof onthe plates 41 and 42, geometrical shapes and magnetic permeability ofthe plates 41 and 42, and the distribution of all the holes on theplates 41 and 42. Hence, the magnetic fields of all the sub-lenses arefundamentally not a pure round-lens field and different from each otherin distribution pattern and strength as shown in FIG. 1B. Consequently,there are two inherent issues which hinder all the electron beams toobtain superior resolutions similar to that of a conventional singlebeam focused by a single-axis lens.

The first issue is a non-axisymmetry of the magnetic field in eachsub-lens. The magnetic field distribution of each sub-lens degeneratesfrom axial symmetry to a rotation symmetry and/or n-fold symmetry. Interms of Fourier analysis, the magnetic field comprises not only anaxisymmetric component or called as round-lens field, but also a lot ofnon-axisymmetric transverse field components or called as high orderharmonics, such as dipole field and quadrupole field. Only theround-lens field is necessary for focusing an electron beam, and theother components are undesired due to their impairment on beam focusing.The dipole field deflects the charged particle beam, thereby making thebeam land on the image plane with position error, additional tilt angleand deflection aberrations, while the quadrupole field adds astigmatismto the beam focusing. To compensate the influence of each high orderharmonic, at least one additional element generating the same type fieldis required for each electron beam.

The second issue is the focusing power differences among all thesub-lenses if all the through round holes are same in geometry. Theround-lens fields of all the sub-lenses are not equal to each other dueto the differences in magnetic flux leakage. The sub-lens closer to thegeometrical center of the plates 41 and 42 has a weaker round-lensfield. For instance, compared with the sub-lenses 10 and 30, thesub-lens 20 has a weaker round-lens field. Due to the round-lens fielddifferences, the beams 1, 2 and 3 respectively passing through thesub-lens 10, 20 and 30 are focused onto different image planes, not asame image plane.

Many scientists propose methods to fundamentally mitigate or eveneliminate the two issues per se. Lo et al. in U.S. Pat. No. 6,750,455uses a plurality of dummy holes to improve the local structure symmetryof each sub-lens. However this method makes the multi-axis magnetic lenssystem bulky. Chen et al. in U.S. Pat. No. 8,003,953 forms apermeability-discontinuity (simply expressed as PD hereafter) unitinside each hole of every sub-lens to eliminate non-axisymmetrictransverse field components inside every sub-lens and the focusing powerdifference among all the sub-lenses. For the sake of clarity, theforegoing unit is named as the first-type PD unit hereafter. Abstractlyspeaking, the first-type PD unit comprises non-magnetic and magneticannular layers in alternate arrangement, i.e. a magnetic annular layeris immediately enclosed by a non-magnetic annular layer and/orimmediately encloses a non-magnetic annular layer. Inside every holewhere a first-type PD unit is formed, the outermost layer adjoins theinner sidewall of the hole, and the innermost layer is a magneticannular layer and becomes a pole-piece of the sub-lens formed by thehole. Concretely speaking, one or more magnetic rings with highpermeability are inserted into each hole of every sub-lens and separatedby a non-magnetic radial gap from each other so as to form multiplecoaxial layers. From the inner sidewall of the hole to the innermostlayer of the unit (i.e. the innermost magnetic ring), permeability atleast alternately decreases and increases spatially one time. Theinnermost magnetic ring is the pole-piece of the sub-lens formed by thehole. FIG. 2A exemplifies a simple embodiment of the first-type PD unit,which takes the sub-lens 30 in FIG 1A as an example and renames it as30-1 for the sake of clarity.

In FIG. 2A, two magnetic rings 32 and 33 both having high permeabilityare respectively inserted into the two coaxial holes in magneticconductor plates 41 and 42 with two radial gaps G1 and G2. The two gapsG1 and G2 are either a vacuum space or filled with a non-magneticmaterial. On the one hand, inside the hole of the plate 41, onefirst-type PD unit is formed by the gap G1 and the magnetic ring 32, andconsequently permeability spatially decreases from permeability u41 ofthe magnetic conductor plate 41 to 1 and then increases to permeabilityu32 of the magnetic ring 32. On the other hand, inside the hole of theplate 42, one first-type PD unit is formed by the gape G2 and themagnetic ring 33, and consequently permeability spatially decreases frompermeability u42 of the magnetic conductor plate 42 to 1 and thenincreases to permeability u33 of the magnetic ring 33. The magneticrings 32 and 33 therefore constitute two pole-pieces of the sub-lens30-1. A magnetic field along the optical axis 31 is generated throughthe non-magnetic gap between these two pole-pieces 32 and 33. The upperpole-piece 32 is extended into the inner hole 33 h of the lowerpole-piece 33 to eliminate the non-axisymmetric transverse fieldcomponents in the gap. The thicknesses of gaps G1 and G2 on the one handhave to be small enough to keep a sufficient magnetic coupling formaking the round-lens field strong enough, and on the other hand largeenough to minimize non-axisymmetric transverse field components to anegligible level inside the inner holes 32 h and 33 h of the upper andlower pole-pieces 32 and 33 respectively. The non-axisymmetrictransverse field components generated outside the sub-lens are reducedby two magnetic tubes 36 and 37. In such a way, the non-axisymmetrictransverse field components in the areas inside and outside eachsub-lens are reduced to a level much lower than that in FIG. 1A. Theround-lens field differences or called as focusing power differencesamong all the sub-lenses are eliminated by specifically choosingthickness differences of the gap G1 and/or the gap G2 among all thesub-lenses.

Based on the fundamental of the multi-axis magnetic lens in U.S. Pat.No. 8,003,953, Chen et al. further propose a multi-axis magneticimmersion objective in the cross-reference, which comprises a pluralityof immersion objective sub-lenses so that a plurality of chargedparticle beams can be individually and in parallel focused onto aspecimen surface with small aberrations. One embodiment is shown in FIG.2B, which also takes the sub-lens 30 in FIG. 1A as an example andrenames it as 30-2 for the sake of clarity. Two magnetic shieldingplates 50 and 51 with a plurality of through round openings sandwich themagnetic conductor plates 41 and 42 with two axial gaps G11 and G12. Themagnetic rings 32 and 33 are respectively inserted into a pair ofcoaxial holes on plates 41 and 42 with two radial gaps G1 and G2 to formtwo first-type PD units therein and become two pole-pieces of thesub-lens 30-2. The gaps G1 and G2 are either a vacuum space or filledwith a non-magnetic material. The lower ends of two pole-pieces 32 and33 are configured to form a radial gap G3 opposite to the specimen 60and extended inside the corresponding coaxial hole in the lower magneticshielding plate 50.

As well known, a new multi-axis magnetic lens is always desired if it iseasier in manufacturing and at least not worse in performance than theprior art. Accordingly, increasing simplicity and flexibility of thefirst-type PD unit in configuration and manufacturing is needed inreducing ease and cost of manufacturing.

SUMMARY OF THE INVENTION

On the basis of the fundamental of U.S. Pat. No. 8,003,953 and thecross-reference, the object of this invention is to provide a multi-axislens with a structure simpler and requiring fewer limitations formanufacture than the prior art. At first, each sub-lens of themulti-axis magnetic immersion objective lens in the cross-reference issimplified to have only one first-type PD unit rather than two, and thenthe simplified lens is further configured to be a multi-axiselectromagnetic compound immersion objective lens. Then, a first-type PDunit is modified to be more flexible in selection of manufacturingmaterial and mechanical structure, and the unit after such amodification is named as a second-type PD unit. Accordingly, a pluralityof second-type PD units is used to configure several types of multi-axismagnetic or electromagnetic compound lens.

In a first embodiment, a multi-axis magnetic immersion objective lens isdisclosed, which comprises a pair of parallel magnetic conductor plateswith a plurality of through round holes in pairs therein, a plurality ofmagnetic rings inside and aligned with the plurality of through roundholes with a plurality of pairs of an upper radial gap and a lowerradial gap respectively, and a common excitation coil located betweenthe pair of parallel magnetic conductor plates. The pair of parallelmagnetic conductor plates includes an upper plate and a lower plate. Foreach paired through round holes, an upper through round hole in theupper plate is aligned with a corresponding lower through round hole inthe lower plate. For each magnetic ring in each paired through roundholes, the upper and lower radial gaps are respectively between outersidewall of said magnetic ring and inner sidewalls of the upper andlower through round holes respectively, thereby forming a plurality ofmagnetic sub-lens modules for focusing a plurality of charged particlebeams respectively. For each sub-lens module, the magnetic ringfunctions as an upper pole-piece and a portion forming the lower throughround hole in the lower plate functions as a lower pole-piece. Thecommon excitation coil is used for providing magnetic flux to theplurality of magnetic sub-lens modules.

A specimen is located below and parallel to the lower magnetic conductorplate. For each magnetic sub-lens module, the magnetic ring has highpermeability. Moreover, the lower radial gap may be vacuum or filledwith non-magnetic material, and the upper radial gap may be vacuum orfilled with non-magnetic material or weakly-magnetic material havingpermeability much smaller than that of both the magnetic ring and thelower magnetic conductor plate. Further, a lower end of the magneticring is configured to coincide with or extend below a bottom surface ofthe lower magnetic conductor plate, and a thickness of the upper radialgap is smaller than a thickness of the lower radial gap and the lowerradial gap has a funnel shape with a narrow lower end opposite to anupper surface of the specimen.

The multi-axis magnetic immersion objective lens according to thepresent invention may further comprise an upper magnetic shielding platelocated above the upper magnetic conductor plate and a lower magneticshielding plate located between the lower magnetic conductor plate andthe specimen, wherein the upper and lower magnetic shielding plates havea plurality of circular openings aligned with the plurality of throughround holes respectively so as to efficiently reduce non-axisymmetrictransverse field components above and below the upper and lower magneticconductor plates respectively. The multi-axis magnetic immersionobjective lens may still comprise a magnetic stage located below thespecimen to sustain the specimen thereon, wherein the magnetic stagemagnetically couples with the upper and lower pole-pieces of eachsub-lens module to create a strong magnetic field immersion on the uppersurface of the specimen.

All the magnetic sub-lens modules are configured to have same focusingpower by using a specific arrangement of thickness differences and/orpermeability differences among the plurality of upper radial gaps. Foreach magnetic sub-lens module, the magnetic ring and the specimen areelectrically excited to act as an electrostatic sub-lens so that everymagnetic sub-lens module can become an electromagnetic compound sub-lensmodule. The multi-axis magnetic immersion objective lens may furthercomprise a plurality of annular electrodes aligned with the plurality ofmagnetic rings respectively, wherein for each electromagnetic compoundsub-lens model, the electrode is configured between the magnetic ringand the specimen to efficiently control electric field on the uppersurface of the specimen. Moreover, the multi-axis magnetic immersionobjective lens may comprise an annular multilayer inside every upperradial gap, wherein said annular multilayer comprises weakly-magneticannular layers and magnetic annular layers in alternate arrangement andan innermost weakly-magnetic annular layer of said annual multilayeradjoins said upper pole-piece. For said annular multilayer, one or moreof said weakly-magnetic annular layers may be replaced by a non-magneticannular layer or vacuum.

In a second embodiment, a multi-axis magnetic lens is disclosed, whichcomprises a pair of parallel magnetic conductor plates with a pluralityof through round holes in pairs therein, a plurality of magnetic ringsin pairs inside and aligned with the plurality of through round holeswith a plurality of first radial gaps in pairs respectively, and acommon excitation coil located between the pair of parallel magneticconductor plates. The pair of parallel magnetic conductor platesincludes an upper plate and a lower plate. For each paired through roundholes, an upper through round hole in the upper plate is aligned with acorresponding lower through round hole in the lower plate. For eachpaired first radial gaps formed by each paired magnetic rings insideeach paired through round holes, a first upper radial gap is between aninner sidewall of the upper through round hole and an outer sidewall ofthe upper magnetic ring, a first lower radial gap is between an innersidewall of the lower through round hole and an outer sidewall of thelower magnetic ring, and one of said first upper and lower radial gapsis filled with weakly-magnetic material with permeability much smallerthan that of the magnetic ring and the magnetic conductor plate.Therefore a plurality of magnetic sub-lens modules is formed forfocusing a plurality of charged particle beams respectively, whereinpaired upper and lower magnetic rings of each sub-lens module functionas upper and lower pole-pieces of said sub-lens respectively. The commonexcitation coil is used for providing magnetic flux to the plurality ofmagnetic sub-lens modules.

For said each magnetic sub-lens module, each of the paired magneticrings has high permeability. Moreover, the other of said each of thepaired first radial gaps is vacuum or filled with non-magnetic materialor weakly-magnetic material with permeability much smaller than that ofthe magnetic ring and the magnetic conductor plate. The multi-axismagnetic lens may further comprise an upper magnetic shielding platelocated above the upper magnetic conductor plate and a lower magneticshielding plate located below the lower magnetic conductor plate,wherein the upper and lower magnetic shielding plates have a pluralityof circular openings aligned with the plurality of through round holesrespectively so as to efficiently reduce non-axisymmetric transversefield components generated above and below the upper and lower magneticconductor plates respectively. All the magnetic sub-lens modules areconfigured to have same focusing power by using a specific arrangementof thickness differences and/or permeability differences among theplurality of paired first radial gaps.

For said each magnetic sub-lens module, a lower end of the upperpole-piece can be extended downward through inside an inner hole of thelower pole-piece to form an axial magnetic-circuit gap which is betweenlower ends of the upper and lower pole-pieces and has a length largerthan thicknesses of the paired first radial gaps, so that a magneticround lens field can be generated through said gap for focusing acharged particle beam. For said each magnetic sub-lens module, an upperend of the lower pole-piece can be extended upward through inside aninner hole of the upper pole-piece to form an axial magnetic-circuit gapwhich is between upper ends of the upper and lower pole-pieces and has alength larger than thicknesses of the paired first radial gaps, so thata magnetic round lens field can be generated through said gap forfocusing a charged particle beam.

A specimen can be located below and parallel to the lower magneticshielding plate. In such a case, for said each magnetic sub-lens module,a lower end of the upper pole-piece can be extended downward throughinside an inner hole of the lower pole-piece to form a radialmagnetic-circuit gap which is between lower ends of the upper and lowerpole-pieces and has a thickness larger than thicknesses of the pairedfirst radial gaps, so that a magnetic round lens field can be generatedthrough said gap for focusing a charged particle beam onto an uppersurface of said specimen with smaller aberrations. For said eachmagnetic sub-lens module, the upper pole-piece and the specimen can beelectrically excited to act as an electrostatic sub-lens so that everymagnetic sub-lens module can become an electromagnetic compound sub-lensmodule. The multi-axis magnetic lens may further comprise a plurality ofannular electrodes aligned with the plurality of upper pole-piecesrespectively, wherein for said each electromagnetic compound sub-lensmodel the electrode is configured between the upper pole-piece and thespecimen to efficiently control electric field on the upper surface ofthe specimen.

For said each magnetic sub-lens module, each of the paired magneticrings has high permeability, and the other of said each of the pairedfirst radial gaps comprises an annular multilayer therein, and saidannular multilayer comprises weakly-magnetic annular layers and magneticannular layers in alternate arrangement and an outermost and aninnermost weakly-magnetic annular layers of said annual multilayerrespectively adjoin an outer and an inner boundaries of said firstradial gap. For said annular multilayer, one or more of saidweakly-magnetic annular layers can be replaced by a non-magnetic annularlayer or vacuum.

In a third embodiment, a multi-axis magnetic lens is disclosed, whichcomprises a pair of parallel magnetic conductor plates with a pluralityof through round holes in pairs therein, a plurality of magnetic ringsin pairs inside and aligned with the plurality of through round holeswith a plurality of first radial gaps in pairs respectively, and acommon excitation coil located between the pair of parallel magneticconductor plates. The pair of parallel magnetic conductor platesincludes an upper plate and a lower plate. For each paired through roundholes, an upper through round hole in the upper plate is aligned with acorresponding lower through round hole in the lower plate. For eachpaired first radial gaps formed by each paired magnetic rings insideeach paired through round holes, a first upper radial gap is between aninner sidewall of the upper through round hole and an outer sidewall ofthe upper magnetic ring, a first lower radial gap is between an innersidewall of the lower through round hole and an outer sidewall of thelower magnetic ring, and one of said first upper and lower radial gapscomprises an first annular multilayer therein. Therefore a plurality ofmagnetic sub-lens modules is formed for focusing a plurality of chargedparticle beams respectively, wherein paired upper and lower magneticrings of each sub-lens module function as upper and lower pole-pieces ofsaid sub-lens respectively. The common excitation coil is used forproviding magnetic flux to the plurality of magnetic sub-lens modules.

Said first annular multilayer comprises weakly-magnetic annular layersand magnetic annular layers in alternate arrangement and an outermostand an innermost weakly-magnetic annular layers of said annualmultilayer respectively adjoin an outer and an inner boundaries of saidfirst radial gap. For said each magnetic sub-lens module, each of thepaired magnetic rings has high permeability, and the other of said eachof the paired first radial gaps can be vacuum or filled withnon-magnetic material or comprise a second annular multilayer therein,and said second annular multilayer comprises weakly-magnetic annularlayers and magnetic annular layers in alternate arrangement and anoutermost and an innermost weakly-magnetic annular layers of said secondannual multilayer respectively adjoin an outer and an inner boundariesof said first radial gap. Further, one or more of said weakly-magneticannular layers of said second annular multilayer can be replaced by anon-magnetic annular layer or vacuum.

For said first annular multilayer, one or more of said weakly-magneticannular layers can be replaced by a non-magnetic annular layer orvacuum, and the other of said each of the paired first radial gaps alsocomprises an annular multilayer therein which is same as said firstannular multilayer having at least one weakly-magnetic annular layer.Moreover, for said first annular multilayer, one or more but not all ofsaid weakly-magnetic annular layers can be replaced by a non-magneticannular layer or vacuum, and the other of said each of the paired firstradial gap is vacuum or filled with non-magnetic material.

In a fourth embodiment, a permeability-discontinuity unit is disclosed,which comprises at least one weakly-magnetic annular layer and at leastone magnetic annular layer, wherein one magnetic annular layer isimmediately enclosed by one weakly-magnetic annular layer and/orimmediately encloses one weakly-magnetic annular layer, and theinnermost annular layer is a magnetic annular layer, whereinpermeability of each weakly-magnetic annular layer is much smaller thanthat of each magnetic annular layer which adjoins said weakly-magneticannular layer.

Any weakly-magnetic annular layer can be replaced by a non-magneticannular layer or vacuum space, and can further comprise more than oneweakly-magnetic sub-layers, while any magnetic layer can furthercomprise more than one magnetic sub-layers. Any weakly-magneticsub-layer of said any weakly-magnetic annular layer can be replaced by anon-magnetic sub-layer.

In a fifth embodiment, a method for decreasing magnetically-couplinginside a hole of a magnetic plate is disclosed, which comprises a stepof providing a magnetic ring which is inside said hole, wherein a radialgap is formed between an inner sidewall of said hole and an outersidewall of said magnetic ring, wherein permeability inside said radialgap is much lower than that of said magnetic plate and said magneticring so that a magnetic flux being leaked from and then being coupledinto inner sidewall of said hole is reduced.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1A is a schematic illustration of a conventional multi-axismagnetic lens.

FIG. 1B is a schematic illustration of magnetic flux lines of themulti-axis magnetic lens shown in FIG. 1A.

FIG. 2A is a schematic illustration of one magnetic sub-lens with twofirst-type PD units proposed in the prior art (U.S. Pat. No. 8,003,953).

FIG. 2B is a schematic illustration of one magnetic immersion objectivesub-lens with two first-type PD units proposed in the cross-reference.

FIG. 3A is a schematic illustration of one magnetic immersion objectivesub-lens in a multi-axis magnetic immersion objective lens in accordancewith one embodiment of the present invention.

FIG. 3B is a schematic illustration of a multi-axis magnetic immersionobjective lens in accordance with the embodiment of the presentinvention shown in FIG. 3A.

FIG. 4 is a schematic illustration of an electromagnetic compoundimmersion objective sub-lens in a multi-axis electromagnetic compoundimmersion objective lens in accordance with another embodiment of thepresent invention.

FIGS. 5A-5D are schematic illustrations of a second-type PD unit inaccordance with another embodiment of the present invention.

FIG. 5E and FIG. 5F are schematic illustrations of a hybrid-type PD unitin accordance with another embodiment of the present invention.

FIG. 6A is a schematic illustration of two second-type PD units in onemagnetic sub-lens of a multi-axis magnetic lens in accordance withanother embodiment of the present invention.

FIG. 6B and FIG. 6C are schematic illustrations of the top views ofthree sub-lenses in the multi-axis magnetic lens in accordance with theembodiment of the present invention shown in FIG. 6A.

FIG. 7A is a schematic illustration of a multi-axis magnetic lens with aplurality of second-type PD units as shown in FIG. 6A in accordance withanother embodiment of the present invention.

FIG. 7B is a schematic illustration of a multi-axis magneticnon-immersion objective lens with a plurality of second-type PD units asshown in FIG. 6A in accordance with another embodiment of the presentinvention.

FIG. 7C is a schematic illustration of a multi-axis magneticnon-immersion objective lens with a plurality of second-type PD units asshown in FIG. 6A in accordance with another embodiment of the presentinvention

FIG. 7D is a schematic illustration of a multi-axis magnetic immersionobjective lens with a plurality of second-type PD units as shown in FIG.6A in accordance with another embodiment of the present invention.

FIG. 8 is a schematic illustration of one magnetic immersion objectivesub-lens in a multi-axis magnetic immersion objective lens in accordancewith another embodiment of the present invention.

FIG. 9 is a schematic illustration of a multi-axis electromagneticcompound immersion objective lens in accordance with another embodimentof the present invention.

FIG. 10A is a first example schematic illustration of one magneticobjective lens for simulation.

FIG. 10B includes three simulation results by three conditions inaccordance with a structure in FIG. 10A.

FIG. 11A is a second example schematic illustration of one magneticobjective lens for simulation.

FIG. 11B includes three simulation results by three conditions inaccordance with a structure in FIG. 11A.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of asub-lens”, while “radial” means “in a direction perpendicular to theoptical axis of a sub-lens”.

In this invention, every multi-axis magnetic lens has a basicconfiguration of a conventional multi-axis magnetic lens as shown inFIG. 1A and 1B. Accordingly, all terms relate to through round holes,circular openings, or circular orifices mean round openings or circularholes penetrated through one plate. Sometimes, especially through roundholes always refer to holes in a magnetic conductor plate, circularopenings always refer to holes in a magnetic shielding plate, andcircular orifices always refer to holes in an electrode plate. For eachsub-lens, upper and lower through round holes respectively refer to thethrough round holes in the upper and lower magnetic conductor plates,and upper and lower pole-pieces respectively refer to the pole-piecesoriginally belong to the upper and lower holes.

In this invention, “weakly-magnetic and magnetic annular layers inalternate arrangement” means that a magnetic annular layer isimmediately enclosed by a weakly-magnetic annular layer and/orimmediately encloses a weakly-magnetic annular layer, and permeabilityof every weakly-magnetic annular layer is much smaller than that ofevery magnetic annular layer which adjoins it.

A conventional multi-axis magnetic lens, as shown in FIG. 1A, basicallycomprises one common excitation coil 44, one yoke 43 and two magneticconductor plates 41 and 42 with a plurality of through round holesrespectively for the corresponding charged particle beams passingthrough. Due to the inherent non-axisymmetric structure of theconventional multi-axis magnetic lens, all the magnetic sub-lensesthereof are not only different from each other in magnetic round-lensfield but also generate many non-axisymmetric transverse fieldcomponents. To make all the sub-lenses function equally and perform aswell as an axisymmetric single-axis lens so as to individually andsimultaneously focus a plurality of charged particle beams onto a sameimage plane with high imaging resolution, the foregoing two issues mustbe solved or eliminated. Accordingly, some additional elements have tobe added.

A solution proposed by Chen et al. is to insert one first-type PD unitinto each hole of every sub-lens of the foregoing multi-axis magneticlens to eliminate non-axisymmetric transverse field components insideevery sub-lens and the focusing power difference among all thesub-lenses. As shown in FIG. 2A, one simple embodiment of a first-typePD unit comprises one non-magnetic gap and one magnetic ring, such asthe magnetic gap G1 and the magnetic ring 32. Furthermore, Chen et al.configure two first-type PD units in each sub-lens to form a multi-axismagnetic immersion objective lens, as shown in FIG. 2B, so as to obtainhigher imaging resolution.

The present invention further configures the multi-axis magnetic lensesproposed by Chen et al. so as to have a structure simpler and to requirefewer limitations for manufacture than before. At first, a multi-axismagnetic immersion objective lens with fewer first-type PD units isprovided, which is then used to configure a multi-axis electromagneticcompound immersion objective lens, as respectively shown in FIG. 3A,FIG. 3B and FIG. 4. The performance of the multi-axis magnetic immersionobjective lens is simulated and compared with the cross-reference, asshown in FIG. 10A and 10B. Secondly, a second-type PD unit is proposedand shown in FIGS. 5A˜5D and FIGS. 6A˜6C, which is more flexible inselection of manufacturing material and mechanical structure. Theperformances of a first-type PD and a second-type PD are simulated andcompared, as shown in FIG. 11A and FIG. 11B. Based on first-type andsecond-type PD units, a hybrid-type PD unit is provided and shown inFIG. 5E and FIG. 5F. Thirdly a plurality of second-type PD units is usedto configure several types of multi-axis magnetic or electromagneticcompound lens, as shown in FIGS. 7A-7D, FIG. 8 and FIG. 9.

The following will describe some embodiments of this invention withreferring to the related drawings. Similarly to the foregoingdescription way, for the sake of clarity, detailed discussions on theembodiment features of the invention are based on a multi-axis magneticlens comprising three magnetic sub-lenses (one at center, two atperiphery) for focusing three electron beams individually, and on theconfiguration of one peripheral sub-lens. However, it would berecognized that every multi-axis magnetic lens proposed in the presentinvention has a basic configuration of a conventional multi-axismagnetic lens and certainly comprises a plurality of sub-lenses withsame configuration. Although the number of sub-lens is free to increase,it is better to locate every new sub-lens with the least increasing ofthe geometric structure's asymmetry of the multi-axis magnetic objectivelens.

Firstly, a multi-axis magnetic immersion objective lens having onefirst-type PD unit in each magnetic sub-lens is provided, and oneembodiment of a peripheral sub-lens 30-3 is shown in FIG. 3A. For thesub-lens 30-3, the first-type PD unit is configured inside the hole inthe upper magnetic conductor plate 41. The magnetic ring 32 with highpermeability is inserted into the upper hole with a radial gap G1 toform the first-type PD unit therein. Inside the upper hole, from theinner sidewall thereof to the magnetic ring 32, permeability spatiallydecreases from permeability u41 of the plate 41 to 1 and then increasesto permeability u32 of the magnetic ring 32. The magnetic ring 32 is theupper pole-piece of the sub-lens 30-3, and the portion forming the holein the lower magnetic conductor plate 42 is the lower pole-piece of thesub-lens 30-3. The lower end of the upper pole-piece 32 is extended intothe inner hole of the lower pole-piece to form a radial magnetic-circuitgap G4 which is opposite to the upper surface of the specimen 60. Thegaps G1 and G4 are either a vacuum space or filled with a non-magneticmaterial, and the thickness of the gap G1 is much smaller than thethickness of the gap G4.

In FIG. 3A, a magnetic field along the optical axis 31 and close to thespecimen 60 is generated through the lower end of the radial gap G4.Both the first-type PD unit and the radial gap G4 are configured to notonly minimize non-axisymmetric transverse field components of themagnetic field to a negligible level but also ensure the round-lensfield component of the magnetic field strong enough. To do so,accordingly, the gap G1 must have an appropriate thickness, and thelower end of the pole-piece 32 is preferred to coincide with or extendbelow the bottom surface of the magnetic conductor plate 42. Moreover,the radial gap G4 is preferred to have a funnel shape whose narrow lowerend opposites to the specimen 60. The specimen stage 61 can be magneticto increase magnetic immersion onto the specimen surface for reducingaberrations. The holes in two magnetic shielding plates 50 and 51 arefor electron beam passing through, but their sizes must small enough soas to efficiently reduce the non-axisymmetric transverse fieldcomponents respectively above and below the upper and lower magneticconductor plates 41 and 42, and larger enough so as to avoid obviousmagnetic coupling with the upper pole-piece 32.

Because the thicknesses of the gaps G1 and G4 influence the magneticcoupling of the upper pole-piece 32 and the magnetic conductor plates 41and 42 in FIG. 3A, either or both in every sub-lens can be appropriatelydesigned to balance the round lens field difference among all thesub-lenses. FIG. 3B shows a multi-axis magnetic immersion objective lens100 with three sub-lenses all having configurations same as shown inFIG. 3A. Each of the three magnetic shielding tubes stacked on the uppermagnetic shielding plate 51, such as 331, is aligned with the opticalaxis of the correspondent sub-lens, and used to further reduce thenon-axisymmetric transverse field components on the path of the electronbeam above the sub-lens.

In addition, the upper pole-piece 32, the magnetic shielding plate 50and the specimen 60 in FIG. 3A can also be electrically excited to actas an electrostatic sub-lens. Consequently, the magnetic sub-lens 30-3can be also functioned as an electromagnetic compound sub-lens, therebyenabling the multi-axis magnetic immersion objective lens 100 in FIG. 3Bto act as a multi-axis electromagnetic compound immersion objective aswell. For the case that the electron beam enters the sub-lens 30-3 withenergy higher than the landing energy on the specimen surface, themagnetic ring 32, the magnetic shielding plate 50 and the specimen 60can be electrically excited to function as an electrostatic retardingsub-lens. In this case, the electron beam begins to decelerate asgradually leaving the magnetic ring 32 and finally to a desired landingenergy on the specimen 60. It is known that the higher the electricfield on the specimen surface, the smaller the focusing aberrations willbe. However, different types of specimen may suffer different electricfield strength. Hence, the electric field strength is preferred to beadjustable without changing landing energy. Adjusting the voltage of themagnetic shielding plate 50 can change the electric field all over thespecimen surface, but such a change is not obvious at the landing placeof the electron beam due to a larger hole 50 h in the magnetic shieldingplate 50. To avoid a strong magnetic coupling of the upper pole-piece 32and the magnetic shielding plate 50, the hole 50 h cannot be smallenough.

This problem can be solved by inserting a non-magnetic annular electrodebetween the upper pole-piece 32 and the specimen 60, and FIG. 4 showsone embodiment. To get smaller aberrations as much as possible, it isnot preferred to additionally increase the distance between the bottomend of the upper pole-piece 32 and the upper surface of the specimen 60.Therefore, the electrode 38 in FIG. 4 is located inside the hole of themagnetic shielding plate 50 and electrically isolated from it. However,the electrode 38 can be below or above the magnetic shielding plate 50.The inner diameter of the annular electrode is appropriately designed sothat adjusting the voltage of the electrode 38 can effectively changethe electric field on the specimen surface.

The performance of the multi-axis magnetic immersion objective lensshown in FIG. 3A is simulated and compared with the cross-reference by asimplified but fundamental structure model without loss of generality,as shown in FIGS. 10A and 10B. FIG. 10A shows one peripheral magneticsub-lens model. Compared with FIG. 3A, the two magnetic shielding plates50 and 51 are not included, the lower hole has a simply cylinder shape,and the gaps G1 and G2 are filled with material 34 and 35. When thepermeability u34 and u35 of the material 34 and 35 are equal to thepermeability u41 and u42 of the magnetic conductor plates 41 and 42, themodel has no first-type PD unit and becomes a conventional case. Whenpermeability u34 and u35 are equal to 1, the model has two first-type PDunits and becomes a case proposed in the cross-reference. When thepermeability u34 and u35 are equal to 1 and the permeability u42respectively, the model has one first-type PD unit and becomes afundamental case shown in FIG. 3A. For the sake of clarity, these threecases are named as Case 1, Case 2 and Case 3 respectively. The opticalaxis 31 is along the Z-direction. Z1 and Z2 denote positions of theupper and lower surfaces of the magnetic conductor plates 41 and 42respectively, Z3 denotes the position of the upper surface of a sample,and WD is the distance between Z2 and Z3 and named as working distance.

The three cases are compared by set u41=u42=5000, and their dipole fielddistributions are shown in FIG. 10B by a dash line, a slim line and abold line respectively. Obviously, it is very efficient that twofirst-type PD units are used to reduce the dipole field generated insidethe multi-axis magnetic sub-lens as mentioned in U.S. Pat. No.8,003,953. Furthermore, the influence of the lower first-type PD unit inCase 2 is negligible if WD is small enough. Therefore Case 3 worksalmost as well as Case 2.

Secondly, a second-type PD unit is proposed, which comprisesweakly-magnetic and magnetic annular layers in alternate arrangement.Inside a second-type PD unit, a magnetic annular layer is immediatelyenclosed by a weakly-magnetic annular layer and/or immediately enclosesa weakly-magnetic annular layer. Further, permeability of everyweakly-magnetic annular layer is much smaller than that of everyadjacent magnetic annular layer, and the innermost layer is a magneticannular layer. When a second-type PD unit is inserted into a hole of amagnetic plate, the outermost layer adjoins the inner sidewall of thehole and the innermost magnetic layer is one pole-piece originallybelonging to the hole. If the outermost layer is a weakly-magneticlayer, permeability thereof is much smaller than that of the magneticplate. From the inner sidewall of the hole to the innermost layer of theunit (i.e. the pole-piece), permeability at least alternately decreasesand increases spatially one time. In addition, the combination of allthe layers except the innermost layer (the pole-piece) is called as amultilayer for the sake of clarity. FIG. 5A and FIG. 5B shows oneembodiment of a second-type PD unit in a top view and a sectional viewrespectively. The second-type PD unit is inserted inside a hole in theupper magnetic conductor plate 41 and comprises four annular layers 32,81, 82, 83. The two magnetic layers 32 and 82 and the twoweakly-magnetic layers 81 and 83 are alternately enclosed one by one inradial direction, and the outermost layer 83 neighbors the innersidewall of the hole. The permeability u81 of the weakly-magnetic layer81 is much smaller than permeability u32 and u82 of the magnetic layers32 and 82 respectively, while the permeability u83 of theweakly-magnetic layer 83 is much smaller than permeability u82 and u41of the magnetic layer 82 and the magnetic conductor plate 41respectively. The innermost layer 32 is the pole-piece belonging to thehole, and the other layers (81, 82 and 83) together form a multilayer 34m.

Any layers in a second-type PD unit can further comprise more than onesub-layers of same type. For example, a magnetic layer can comprise twomagnetic sub-layers, and a weakly-magnetic layer can comprise threeweakly-magnetic sub-layers. Furthermore, sub-layers of a magnetic orweakly-magnetic layer can stack one by one or enclose one by one. InFIG. 5C, the weakly-magnetic layer 83 and magnetic layer 82 comprise twoweakly-magnetic sub-layers and two magnetic sub-layers respectively,wherein all the sub-layers stack one by one along the optical axis 31and thereby being called as axial sub-layers. In FIG. 5D, twoweakly-magnetic sub-layers of the weakly-magnetic layer 83 and twomagnetic sub-layers of the magnetic layer 82 respectively enclose one byone in every radial direction and thereby being called as radialsub-layers.

One or more of weakly-magnetic layers in a second-type PD unit can bereplaced by a non-magnetic layer. For example, the weakly-magneticlayers 83 in FIG. 5B can be replaced by a non-magnetic layer 83 n asshown in FIG. 5E. Furthermore, one or more of weakly-magnetic sub-layersof a weakly-magnetic layer in a second-type PD unit can be replaced by anon-magnetic sub-layer. FIG. 5F shows that one weakly-magnetic sub-layerof the weakly-magnetic layer 83 in FIG. 5D is replaced by a non-magneticsub-layer. The second-type PD unit after either or both of the foregoingtwo kinds of replacement is called as a hybrid PD unit.

FIG. 6A shows one simple embodiment of a second-type PD unit, which isinserted inside each hole of the sub-lens 30-1 f. For the upper hole inthe upper magnetic plate 41, the second-type PD unit comprises theweakly-magnetic ring 34 and the magnetic ring 32, and permeability u34of the weakly-magnetic ring 34 is much smaller than permeability u32 ofthe magnetic ring 32 and permeability u41 of the magnetic plate 41. Inthe same way, the second-type PD unit inside the lower hole in the lowermagnetic plate 42 comprises the weakly-magnetic ring 35 and the magneticring 33, and permeability u35 of the weakly-magnetic ring 35 is muchsmaller than permeability u33 of the magnetic ring 33 and permeabilityu42 of the lower magnetic plate 42. The magnetic rings 32 and 33 arerespectively the upper and lower pole-pieces of the sub-lens 30-1 f. Amagnetic field along the optical axis 31 is generated through thenon-magnetic gap between these two pole-pieces 32 and 33. Thethicknesses of the weakly-magnetic rings 34 and 35 are small enough tokeep a sufficient magnetic coupling for making the round-lens fieldstrong enough, and large enough to minimize non-axisymmetric transversefield components to a negligible level inside the holes 32 h and 33 h ofthe upper and lower pole-pieces 32 and 33 respectively. The sub-lens30-1 f in FIG. 6A has a second-type PD unit in each hole; however it canhas a second-type PD unit in one hole and a first-type PD unit in theother hole.

The performances of a first-type PD unit and a second-type PD unit aresimulated and compared by a simplified but fundamental structure modelwithout loss of generality, as shown in FIGS. 11A and 11B. The oneperipheral magnetic sub-lens model is shown in FIG. 11A, which comprisetwo magnetic conductor plates 41 and 42 and two magnetic rings 32 and 33respectively inserted inside two holes in the magnetic conductor plates41 and 42 with two radial gaps. The radial gaps are filled with thematerial 34 and 35. When the permeability u34 and u35 of the material 34and 35 are equal to the permeability u41 and u42 of the magneticconductor plates 41 and 42, the model has no PD unit and becomes aconventional case. When permeability u34 and u35 are equal to 1, themodel has two first-type PD units and becomes a case proposed in theU.S. Pat. No. 8,003,953. When the permeability u34 and u35 are equal to10, the model has two second-type PD units and becomes a fundamentalcase shown in FIG. 6A. For the sake of clarity, these three cases arenamed as Case 4, Case 5 and Case 6 respectively. The optical axis 31 isalong the Z-direction. Z1 and Z2 denote positions of the upper and lowersurfaces of the magnetic conductor plates 41 and 42 respectively.

The three cases are compared by set u41=u42=5000, and their dipole fielddistributions are shown in FIG. 11B by a dash line, a slim line and abold line respectively. Obviously, it is very efficient that twofirst-type PD units are used to reduce the dipole field generated insidethe multi-axis magnetic sub-lens as mentioned in U.S. Pat. No.8,003,953. Furthermore, for each second-type PD unit in Case 3, itsfunction of eliminating dipole field inside the magnetic sub-lens isweakened by its weakly-magnetic layer. However, if the impact is keptdown to a negligible or acceptable degree for an application, Case 3works almost as well as Case 2.

Compared with a first-type PD unit, a second-type PD unit is a littleweaker in efficiently eliminating non-axisymmetric transverse fieldcomponents of a magnetic sub-lens, but provides more flexibility notonly in efficiently eliminating focusing power difference among all thesub-lenses but also in manufacturing. Similarly to the way used forfirst PD unit, FIG. 6B shows one embodiment which uses thicknessdifferences among three weakly-magnetic layers to eliminate focusingpower differences among three sub-lenses (10-1 f, 20-1 f and 30-1 f). InFIG. 6B, three weakly-magnetic layers (14, 24 and 34) inside the threeholes in the upper magnetic conductor plate 41 are same in permeability(u14, u24 and u34) but different in thickness. The two weakly-magneticlayers 14 and 34 respectively in peripheral sub-lenses 10-1 f and 30-1 fhave a same thickness which is thicker than that of the weakly-magneticlayer 24 in the center sub-lens 20-1 f. Besides, FIG. 6C shows anotherembodiment which uses permeability differences among threeweakly-magnetic layers to eliminate focusing power differences amongthree sub-lenses (10-1 f, 20-1 f and 30-1 f). In FIG. 6C, all theweakly-magnetic layers (14, 24 and 34) are same in thickness butdifferent in permeability (u14, u24 and u34). The two weakly-magneticlayers 14 and 34 respectively in peripheral sub-lenses 10-1 f and 30-1 fhave an equal permeability which is lower than that of theweakly-magnetic layer 24 in the center sub-lens 20-1 f. In addition, dueto using weakly-magnetic layers instead of non-magnetic layers, asecond-type PD unit has a broader variety in material selection and bothdesigning and manufacturing of mechanical structure. In a first-type PDunit, every non-magnetic annular layer is either a vacuum gap or made ofa material of non-magnetic conductor. Keeping a vacuum gap raises muchdifficulty for designing and assembling of the unit, and filling a layerwith a perfect non-magnetic material narrows manufacturing materialselection.

Thirdly, a plurality of second-type PD units or both first-type andsecond-type PD units can be used to configure several types ofmulti-axis magnetic or electromagnetic compound lens. FIG. 7A shows amulti-axis magnetic lens 200 for general application such as being usedas a condenser lens or a transfer lens in a multi-beam apparatus. Foreach sub-lens in this multi-axis magnetic lens, a second-type PD unitand a first-type PD unit are respectively configured inside the holes inthe upper and lower magnetic conductor plates 41 and 42. For instance,for the right sub-lens, the second-type PD unit is formed by themagnetic ring 32 and the weakly-magnetic ring 34, the first-type PD unitis formed by the magnetic ring 33 and the radial non-magnetic gap G2.The magnetic rings 32 and 33 respectively become the upper and lowerpole-pieces of the sub-lens, and both are configured to form an axialnon-magnetic gap. A magnetic field along the optical axis 31 isgenerated through the axial non-magnetic gap. To eliminatenon-axisymmetric transverse field components of the magnetic field, theaxial non-magnetic gap is preferred inside the inner hole of one of theupper and lower pole-pieces 32 and 33. FIG. 7A shows the axialnon-magnetic gap inside the inner hole of the lower pole-piece 33, whileFIG. 7C shows the axial non-magnetic gap inside the inner hole of theupper pole-piece 32. The non-axisymmetric transverse field components ofthe magnetic field on the path of the electron beam above and below thesub-lens are eliminated by the upper and lower magnetic tubes 331 and332 which are aligned with the optical axis 31 of the sub-lens. Theround-lens field differences among the three sub-lenses can beeliminated by specifically choosing the thickness differences among thenon-magnetic gaps or the weakly-magnetic rings and/or the permeabilitydifferences among the weakly-magnetic rings.

FIG. 7B shows a multi-axis magnetic non-immersion objective lens 300-1,which is basically similar to the multi-axis magnetic lens 200 in FIG.7A but remove all of the lower magnetic shielding tubes thereof and putthe specimen 60 under the lower shielding plate 50. For each sub-lens inthis multi-axis magnetic lens, two second-type PD units are respectivelyconfigured inside the two holes in the upper and lower magneticconductor plates 41 and 42. For example, for the right sub-lens, theupper second-type PD unit is formed by the magnetic ring 32 and theweakly-magnetic ring 34, the lower second-type PD unit is formed by themagnetic ring 33 and the weakly-magnetic ring 35. The magnetic rings 32and 33 are the upper and lower pole-pieces of the sub-lens, and both areconfigured to form an axial non-magnetic gap inside the inner hole ofthe lower pole-piece 33. A magnetic field along the optical axis 31 isgenerated through the axial non-magnetic gap. To reduce the imagingaberrations of the sub-lens as much as possible, the axial non-magneticgap is formed very close to the specimen 60. In addition, the upper andlower pole-pieces in each sub-lens and the specimen 60 can also beelectrically excited to act as an electrostatic sub-lens. Consequently,each magnetic sub-lens can be also functioned as an electromagneticcompound sub-lens, and thereby enabling the multi-axis magneticnon-immersion objective lens 300 to act as a multi-axis electromagneticcompound non-immersion objective as well. Furthermore, for theapplication that the electron beam enters each sub-lens with energyhigher than the landing energy on the specimen surface, the upper andlower pole-pieces and the specimen 60 can be electrically excited tofunction as an electrostatic retarding sub-lens. For some applicationsthat a lower magnetomotive force (product of turns and current of thecommon excitation coil) is most preferred, the axial non-magnetic gap ofevery sub-lens can be formed a little far away from the specimen 60, oreven inside the inner hole of the upper pole-piece 32 such as themulti-axis magnetic non-immersion objective lens 300-2 shown in FIG. 7C.

FIG. 7D shows a multi-axis magnetic immersion objective lens 400. Foreach sub-lens in this multi-axis magnetic lens, a first-type PD unit anda second-type PD unit are respectively configured inside the holes inthe upper and lower magnetic conductor plates 41 and 42. For instance,for the right sub-lens, the first-type PD unit is formed by the magneticring 32 and the radial non-magnetic gap G1, the second-type PD unit isformed by the magnetic ring 33 and the weakly-magnetic ring 35. Themagnetic rings 32 and 33 respectively are the upper and lowerpole-pieces and configured to form a radial non-magnetic gap inside theinner hole of the lower pole-piece 33. To reduce the imaging aberrationsof the sub-lens as much as possible, the radial axial non-magnetic gapis formed very close to the specimen 60. A magnetic field along theoptical axis 31 is generated through the radial non-magnetic gap andstrongly immerses the surface of the specimen 60. In addition, for theapplication that the electron beam enters each sub-lens with energyhigher than the landing energy on the specimen surface, the uppermagnetic pole-piece 32 and the specimen 60 can be electrically excitedto function as an electrostatic retarding sub-lens. Consequently, eachmagnetic sub-lens can be also functioned as an electromagnetic compoundsub-lens, and thereby enabling the multi-axis magnetic immersionobjective lens 400 to act as a multi-axis electromagnetic compoundimmersion objective as well.

For the multi-axis magnetic immersion objective lens and the multi-axiselectromagnetic compound immersion objective lens shown in FIG. 3A, FIG.3B and FIG. 4, the first-type PD unit in every sub-lens can be replacedby a second-type PD unit. Accordingly, one embodiment of a peripheralsub-lens 30-3 f is shown in FIG. 8. For the sub-lens 30-3 f, thesecond-type PD unit is configured inside the hole in the upper magneticconductor plate 41 and comprises the magnetic ring 32 and theweakly-magnetic ring 34. The magnetic ring 32 is the upper pole-piece ofthe sub-lens 30-3 f, and the portion forming the hole in the lowermagnetic conductor plate 42 is the lower pole-piece of the sub-lens 30-3f. The lower end of the upper pole-piece 32 is extended into the innerhole of the lower pole-piece to form a radial magnetic-circuit gap G4which is opposite to the upper surface of the specimen 60. The gap G4 iseither a vacuum space or filled with a non-magnetic material, and has athickness larger than that of the weakly-magnetic ring 34. A magneticfield along the optical axis 31 and close to the specimen 60 isgenerated through the narrow lower end of the radial gap G4. Similar tothe sub-lens 30-3 shown in FIG. 3A, the upper pole-piece 32, themagnetic shielding plate 50 and the specimen 60 in FIG. 8 can also beelectrically excited to act as an electrostatic sub-lens. Furthermore,an annular electrode can be inserted between the upper pole-piece 32 andthe specimen 60 to efficiently control the electric field on thespecimen surface 60. Such a configuration can be used to form amulti-axis electromagnetic compound immersion objective lens, such as500-ME shown in FIG. 9. In FIG. 9, the annular electrode of everysub-lens, such as the electrode 38 of the right sub-lens, is locatedinside the hole of the magnetic shielding plate 50 and electricallyisolated from it.

In summary this invention provides more types of multi-axis magneticlens on the basis of fundamental of a first-type PD unit proposed byChen et al. in U.S. Pat. No. 8,003,953, which have simpler structuresand require fewer limitations for manufacture than before. At first, amulti-axis magnetic immersion objective lens using only one first-typePD unit in every sub-lens is provided. Reducing one first-type PD unitin every sub-lens obviously simplifies complexity in mechanicalstructure, and thereby good for manufacturing. Secondly a new type PDunit, i.e. second-type PD unit, is proposed. Compared with a first-typePD unit, although it is a little weaker in efficiently eliminatingnon-axisymmetric transverse field components of a magnetic sub-lens,however, if this difference is negligible or acceptable, it providesmore flexibility both in efficiently eliminating focusing powerdifference among all the sub-lenses and in manufacturing such asmaterial selection and mechanical structure. Thirdly a plurality ofsecond-type PD units or a plurality of pairs of a first-type PD unit anda second-type PD unit is used to configure several types of multi-axismagnetic lens. One type is for general application such as a multi-axismagnetic condenser lens or a multi-axis magnetic transfer lens, anothertype is a multi-axis magnetic non-immersion objective which can requirea lower magnetomotive force, and one more type is a multi-axis magneticimmersion objective lens which can generate smaller aberrations. Due tousing first-type and/or second-type PD units, every multi-axis magneticlens in this invention can also be electrically excited to function as amulti-axis electromagnetic compound lens so as to further reduceaberrations thereof and realize electron beam retarding for low-voltageirradiation on specimen.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

What is claimed is:
 1. A multi-axis magnetic immersion objective lens,comprising: a pair of parallel magnetic conductor plates with aplurality of through round holes in pairs therein, the pair of parallelmagnetic conductor plates including an upper plate and a lower plate,for each paired through round holes, an upper through round hole in theupper plate aligned with a corresponding lower through round hole in thelower plate; a plurality of magnetic rings inside and aligned with theplurality of through round holes with a plurality of pairs of an upperradial gap and a lower radial gap respectively, wherein for eachmagnetic ring in each paired through round holes, the upper and lowerradial gaps are respectively between outer sidewall of said magneticring and inner sidewalls of the upper and lower through round holesrespectively, thereby forming a plurality of magnetic sub-lens modulesfor focusing a plurality of charged particle beams respectively, whereinfor each sub-lens module, the magnetic ring functions as an upperpole-piece and a portion forming the lower through round hole in thelower plate functions as an lower pole-piece; and a common excitationcoil located between the pair of parallel magnetic conductor plates forproviding magnetic flux to the plurality of magnetic sub-lens modules,wherein a first lower radial gap of the plurality of pairs of upperradial and lower radial gaps has a funnel shape with a narrow lower endopposite to an upper surface of the specimen.
 2. The multi-axis magneticimmersion objective lens according to claim 1, wherein a specimen islocated below and parallel to the lower magnetic conductor plate.
 3. Themulti-axis magnetic immersion objective lens according to claim 2,wherein for each magnetic sub-lens module, the magnetic ring has highpermeability, and the lower radial gap is vacuum or filled withnon-magnetic material.
 4. The multi-axis magnetic immersion objectivelens according to claim 3, wherein for each magnetic sub-lens module,the upper radial gap is vacuum or filled with non magnetic material orweakly-magnetic material having permeability much smaller than that ofboth the magnetic ring and the lower magnetic conductor plate.
 5. Themulti-axis magnetic immersion objective lens according to claim 4,wherein for each magnetic sub-lens module, a lower end of the magneticring is configured to coincide with or extend below a bottom surface ofthe lower magnetic conductor plate.
 6. The multi-axis magnetic immersionobjective lens according to claim 5, wherein for each magnetic sub-lensmodule, thickness of the upper radial gap is smaller than thickness ofthe lower radial gap.
 7. The multi-axis magnetic immersion objectivelens according to claim 6, further comprising an upper magneticshielding plate located above the upper magnetic conductor plate and alower magnetic shielding plate located between the lower magneticconductor plate and the specimen, wherein the upper and lower magneticshielding plates have a plurality of circular openings aligned with theplurality of through round holes respectively so as to efficientlyreduce non-axisymmetric transverse field components above and below theupper and lower magnetic conductor plates respectively.
 8. Themulti-axis magnetic immersion objective lens according to claim 7,further comprising a magnetic stage located below the specimen tosustain the specimen thereon, wherein the magnetic stage magneticallycouples with the upper and lower pole-pieces of each sub-lens module tocreate a strong magnetic field immersion on the upper surface of thespecimen.
 9. The multi-axis magnetic immersion objective lens accordingto claim 6, wherein all the magnetic sub-lens modules are configured tohave same focusing power by using a specific arrangement of thicknessdifferences among the plurality of upper radial gaps.
 10. The multi-axismagnetic immersion objective lens according to claim 6, wherein for eachmagnetic sub-lens module, the magnetic ring and the specimen areelectrically excited to act as an electrostatic sub-lens so that everymagnetic sub-lens module can become an electromagnetic compound sub-lensmodule.
 11. The multi-axis magnetic immersion objective lens accordingto claim 10, further comprising a plurality of annular electrodesaligned with the plurality of magnetic rings respectively, wherein eachelectrode is configured between each magnetic ring and the specimen toform an electromagnetic compound sub-lens model with the magnetic ringand the specimen to efficiently control electric field on the uppersurface of the specimen.
 12. The multi-axis magnetic immersion objectivelens according to claim 6, wherein all the magnetic sub-lens modules areconfigured to have same focusing power by using a specific arrangementpermeability differences-among the plurality of upper radial gaps. 13.The multi-axis magnetic immersion objective lens according to claim 1,further comprising an annular multilayer inside every upper radial gap,wherein said annular multilayer comprises weakly-magnetic annular layersand magnetic annular layers in alternate arrangement.
 14. The multi-axismagnetic immersion objective lens according to claim 13, for saidannular multilayer, one or more of said weakly-magnetic annular layersis replaced by a non-magnetic annular layer or vacuum.